The purpose of this protocol is to investigate in vivo low density lipoprotein (LDL) and high density lipoprotein (HDL) metabolism in patients with homozygous familial hypercholesterolemia (FH) before and after liver-directed LDL receptor gene therapy. Homozygous FH is a serious genetic disease caused by mutations in the LDL receptor gene and results in markedly elevated LDL levels, decreased HDL levels, and severe premature coronary heart disease. A clinical protocol has been developed to treat patients by ex vivo liver-directed somatic gene transfer of the LDL receptor gene. The purpose of this protocol is to use well-established lipoprotein kinetic methodology to determine the effect of liver-directed LDL receptor gene transfer on LDL and HDL kinetics.